In superconductor magnets, variations in magnetic fields are common occurrences which often cause conduction faults in superconducting filaments. To compensate for such conducting faults, the magnetic windings are usually formed of wires in which the filaments of superconducting material are clad in a stabilizer metal, such as silver, copper or aluminum, which is normally conductive across a wide range of temperatures, including superconducting temperatures. The metal electrically stabilizes the wire by shunting any portion of the filament(s) which has become nonconducting. Furthermore, the encasing metal has good thermal conductivity, tending to transfer heat away from hot spots in the windings. Nevertheless, events, such as flux jumps, wire motion or eddy currents in the encasing metal produce heat which may in certain circumstances lead to a precipitous normalization of superconductive windings. Such temperature excursions occur during periods measured in microseconds, such short periods often being insufficient for dissipation of the heat to the coolant, e.g., liquid helium.
To reduce the probability of normalizing temperature excursions, U.S. Pat. No. 4,171,464 proposes that particles or fibers of gadolinium oxide or gadolinium-aluminum oxide be incorporated in the metal, e.g., copper, that encases the superconducting filaments. These materials have high heat capacities at superconducting temperatures, e.g., below about 5.degree. K., and thus tend to absorb locally produced heat.
Incorporation of heat-absorbing particulates in the stabilizing metal is advantageous in that a large interface is produced between the metal stabilizer and the high heat capacity material. On the other hand, incorporation of the high heat capacity material into the stabilizing metal presents some problems. To begin with, inclusion of high heat capacity material, in either fiber or particulate form, adds to the expense of forming the superconductor.
The inclusion of heat-absorbing material in the metal stabilizer reduces the conductivity of the metal stabilizer in at least two ways, thereby making the metal a less efficient electrical stabilizer. First of all, the high heat capacity heat-absorbing material is substantially nonconducting, and by occupying volume which would otherwise be occupied by the metal, the shunting effect of the stabilizer is reduced. Secondly, trace solubilization of the heat-absorbing material into the stabilizer metal reduces the conductivity of the metal. In superconductors, very pure metals are used as stabilizers so as to have maximum conductivity. Although the heat-absorbing material is included in the stabilizer metal in insolubilized form, the process by which heat-absorbing material is incorporated in the stabilizer metal generally results in some trace solubilization of the material into the metal, which trace solubilization significantly affects the electrical characteristics of the stabilizer metal.
The need continues for more effective means to thermally stabilize superconductors.